For many years designers have strived to provide the preceding qualities in their hulls, and the "deep V" hull, the catamaran or tunnel hull and the hydrofield designs have evolved. However, in gaining the ride softness, the designs have always been a trade-off to degrading some other quality. In the case of the "deep V", increased draft and lack of stability at rest, the catamaran has increased cost of manufacture, increased draft and requires twin powerplants if heavy loads are carried, and the hydrofield requires a narrow beam and shows a lack of buoyancy in the bow.
The development of understanding of planing hulls has taken place only in the last 40 years, and the basic authority is a book entitled "Naval Architecture of Planing Hulls" by Dr Lindsay Lord (Cornell Maritime Press, Inc, Cambridge, Md. USA). On page 31 of the Third Edition of said book there is a reference to the lifting forces and the suction forces which exist in a planing hull, and it is stated (apparently correctly) that it is the change of directional momentum of water particles striking the plane that kinetic energy is transferred. In terms of useful work, lift occurs only at those points where kinetic energy is being impressed upon the water. The amount of lift at these points will vary approximately as the cosine of the true planing angle at a given point.
The most commonly used planing hull has a V-shaped bottom with smooth surfaces and the dead rise angle rapidly increases at the forefoot end of the bottom. Accordingly in many instances, under normal usage, water will flow upwardly over the planing surfaces at the forefoot end, and although that flow will cause a considerable degree of lift, it will cause an upwardly directed bow wave which necessarily results in loss of lift and wastes energy. Furthermore the bow wave on each side of the central longitudinal plane of the bottom will cooperate with the bow wave on the other side to define an angle which may be in the vicinity of 90.degree. and is sometimes obtuse so that the lifting pressure rapidly diminishes rearwardly of the forefoot.
Referring further to the Lord textbook, on page 63 of said edition, reference is made to the forebody sections, and the advantages and disadvantages of various shapes are discussed. The effect of dead rise is discussed and further advantages discussed with respect to lateral plane on page 67, where it is pointed out that a hull suited to blue water cruising has a "more generous lateral plane, distributed fore and aft throughout the underbody".
These considerations led Lord to the conclusion that monohedron lines were the most desirable and that running lines shall be "straight and parallel with each other". The development since that time has been based upon the accurate statements of Lord which can be shown to be correct, but there are necessarily compromises which are reached in the design, and any advantages, for example due to the cross-sectional shape of the hull bottom is accompanied by disadvantages. For example, by having the shape of the hull bottom to comprise a pair of downwardly concave planing surfaces, the water displaced by the hull can be redirected downwards by the curvature and thereby increase the available lift, but hulls which are constructed to this shape pound (that is, hydraulically bottom) and produce extremely high shock loading on to the hull structure upon encountering rough water and there is also more tendency for them to broach than with the more conventional convex hull shapes.
By increasing deadrise into a deep "V" shape, the lateral plane is increased quite effectively and the steeply sloping plane surfaces of the bottom upon tilting result in a correcting moment which is much higher than with bottoms of small deadrise (say 10.degree.) but there is an increased draft and also an instability when the boat is at anchor and does not have the benefit of the dynamic righting forces. An object therefore of this invention is to provide a hull shape which will provide required softness of ride without the instability at anchor and without unnecessarily large lateral planes.
The statements contained in the Lord textbook relating to parallel running lines are obviously correct, but in this invention parallelism is either reduced or completely obviated and consequently some of the benefits due thereto are lost, but experience and tank tests have indicated that the compromise is much in favour of this invention, that is, there is much improvement in the softness of ride for a very small cost in the planing efficiency.